This invention relates to testing apparatus for dynamoelectric machines, and in particular, to a test system for checking the resistance values of the windings of an electric motor. While the invention is described with particular detail with respect to electric motor windings, those skilled in the art will recognize the wider applicability of the inventive concepts disclosed hereinafter.
It has long been the practice in the manufacturing of dynamoelectric machines, and in particular, of electric motors, to run a series of operational and quality control checks to ensure that products produced by a motor plant, for example, are operating properly prior to shipment to a particular customer. One of the more common production tests imposed on windings during manufacture is a winding resistance check. That is to say, a voltage is placed across the motor winding and a determination of the winding resistance is made. Winding resistance is a particularly critical indicia of quality control in a motor plant because the amount of copper or similar material in a motor, from which the winding is made, can seriously affect operating performance of the motor in operational use. It is also a critical check in the performance of other devices used in manufacturing motors. For example, an indication of continued high resistance in the windings of dynamoelectric machines being tested is an indication that the wire of the windings is being stretched during motor manufacture because of a malfunction of the winding machines. In like manner, resistance deviations from a standard resistance design for a particular winding in a motor can be an indication that the turn count of the windings is incorrect, that the wire size of the winding is incorrect, or that other physical damage has occurred to the winding during manufacture. Any of these defects can cause rejection of the motor and it is desirable to discover such defects as quickly as possible during manufacture. Consequently, the stator assembly of a motor is checked relatively soon after insertion of the winding and generally after forming of the end turns of the winding. The winding also can be checked at later points in the production process.
As will be appreciated by those skilled in the art, resistance checking is merely one of a variety of tests performed by electric motor manufacturers to ensure that products produced by a motor plant, for example, are operating properly prior to shipment to a particular customer. Among the more common other tests imposed on motors during manufacture are a surge test in which a high voltage surge is applied to the motor windings to check for wire damage that can result in a open circuit of the motor winding, a high pot or high potential test to check the motor windings for electrical shorts between windings and to ground; and various start tests to ensure the motor will start some predetermined load even under low voltage conditions.
In the past motor winding resistances have been checked in number of ways. For example, a "lab standard" or motor having the desired performance has been used to compare the test characteristics of a production part against the test characteristics of the lab standard. Use of lab standard motors in large production facilities has a number of serious drawbacks. For example, in high volume production facilities, a number of resistance test stations at a single location are employed so that the resistance check does not cause slow down in the production capability of the facility. Where resistance is checked against a lab standard, there are continuing problems in ensuring that the lab standard is the correct one for the particular motor under test, that the standard itself does not become damaged by handling during test, and that a sufficient number of equally matched standards are available for tests. With the lab standard type of test, ambient temperatures of the motor plant is not a consideration as both the standard and the motor under test are at the same temperature.
Because of the difficulty in maintaining a supply of standard motors and the various other problems discussed above, motor manufacturers have resorted to the use of resistance test devices in which the desired value of the resistance is set by means of a decade resistance box, for example. This kind of test equipment is highly desirable because lists of products of a motor plant, together with the proper resistance values for the windings of those products can be maintained either at the machine or with responsible personnel, and the proper resistance set into the machine quickly without the necessity of either maintaining a supply of standard motors or keeping the motors available at the machine. Consequently, the resistance test area of a plant can be operated independently of various production lines. These later types of resistance test devices, however, have heretofore not been provided with means for automatically changing the desired resistance value set into the machine.
It will be appreciated by those skilled in the art that winding resistances generally are calculated for a room ambient temperature of 25.degree. C. Temperatures in motor manufacturing plants have rather large swings. Variations in the range of 20.degree.-30.degree. F. are not uncommon. Commonly, windings for motors are designed to a tolerance of plus or minus five percent of a desired or known standard value of motor winding resistance. A temperature swing of 20.degree.-30.degree. F. can cause the rejection of acceptable motor windings because changes in ambient temperature cause the measured value to exceed the set limits. One solution to this problem has been to employ an individual who periodically checks the temperature in the motor manufacturing plant and adjusts the values set into the resistance checking devices as the temperature varies. While this procedure will give adjusted resistance values, it too is subject to error, often because the adjustments are not made properly.
The invention described hereinafter overcomes these known deficiencies by providing a test device which includes an ambient temperature sensor. The sensor develops an electrical signal which is utilized to correct the standard resistance input into the test unit. The unit also automatically determines a new range of tolerance limits about the desired standard point at the elevated temperature and displays that information to the operator of the device. Since correction occurs automatically, the resistance testing devices of this invention always are set for the proper resistance value during motor test.
One of the objects of this invention is to provide a motor test system which facilitates resistance testing of a motor at a variety of ambient temperature conditions.
Another object of this invention is to provide a motor test system which automatically compensates a known standard resistance value for a motor winding to a value at a particular ambient temperature for the part under test.
Another object of this invention is to provide a motor test system which displays a recalculated value high and low limit about that value of resistance based on ambient temperature conditions for a particular winding resistance under test.
Another object of this invention is to provide a low cost method for automatically adjusting a standard value for dynamoelectric machine winding resistance during production testing of that resistance.
Other objects of this invention will be apparent to those skilled in the art in light of the following description and accompanying drawings.